Headrest guide-tube with interlacing

ABSTRACT

Guide-tubes for supporting the headrest of an automobile seat are lock-beaded into the top-rail of the seat-frame. The guide-tubes are formed from pieces of sheet metal, folded over to form cylindrical tubes. The edges of the sheet are interlaced, by means of dovetailing tabs and sockets, which are stamped into the side-edges of the sheet. The interlacing tabs and sockets are not present over the middle portion of the guide-tube, where the lock-beads are formed.

BACKGROUND TO THE INVENTION

[0001] Automobile headrests have to be attached into the seat strongly enough to provide support in an accident. The headrest is supported from the seat-frame by two pillars, the pillars being insertable into respective guide-tubes, which are integral with the seat-frame. In some cases, the pillars, once inserted into the guide tube, remain fixed relative to the guide-tube; in other cases, the pillars are adjustable for height, within and relative to the guide-tubes.

[0002] Besides ensuring that the headrest supports are strong enough, the designer also needs to consider the fit of the headrest pillars relative to the guide-tubes in the seat-frame. The headrest is not assembled to the seat until the seat is otherwise completed, and it is very unfortunate if the headrest does not fit properly, because the upholstered seat then has to be reworked, or scrapped. So, it is important that, after the guide-tubes have been fixed into the seat-frame, but before the seat-frame is installed in the seat, that a production inspector can easily tell, from an inspection that can be easily carried out at this time, whether the guide-tubes have been fixed properly into the seat-frame, whether the headrest will fit, and whether it will operate properly.

[0003] The designer has to ensure that the position of the two guide-tubes in the seat-frame is correct, bearing in mind the considerable tolerance that must be allowed on the spacing of the two pillars. Also, the designer must consider the fit of each pillar within its respective guide-tube.

[0004] It is customary for moulded plastic sleeves to be placed in the guide-tubes, and for the headrest pillars to be inserted into the sleeves. The resulting fit of the pillars relative to the guide-tubes must be loose enough that assembly is simple, and enough that the pillars will move readily, in the sleeves, relative to the guide-tubes, for adjustment purposes. On the other hand, the fit of the pillars in the guide-tubes should be tight enough to hold the headrest in its adjusted position, and also the fit should be tight enough to eliminate any possibility of rattling, since rattles in the headrest area are intolerable.

[0005] In some cases, the height adjustment of the headrest involves the use of a catch, which is provided in the plastic sleeve. Normally, the catch grips the pillar, but the catch is operated by the seat occupant, to allow the pillar to be adjusted for height. Only one of the sleeves has the catch, so the sleeves have to be left/right handed to ensure the catch-sleeve lies on the right, or on the left, as required, and the guide-tubes have to be complementarily left/right handed. Furthermore, the designer has to make sure the sleeves, as installed in the guide-tube, are orientated correctly (so that the catch faces towards the driver's hand), which in turn means that the guide-tubes as installed in the seat-frame have to be orientated correctly. Ensuring the correct orientation between the guide-tube and the plastic sleeve is often done by means of slots cut in the top end of the guide-tube, which are engaged by pegs moulded onto the sleeves.

[0006] In considering how to provide a structure that will perform in accordance with the above considerations, the designer is also concerned to provide the required performance at a minimum cost.

THE INVENTION IN RELATION TO THE PRIOR ART

[0007] It is known for headrest guide-tubes to be made of welded steel tubing. In this case, two lengths of the welded tubing are welded to the top-rail of the set frame. Welding the guide-tubes to the seat-frame is labour-intensive, and therefore expensive, and the welded joints have to be considerably over-engineered to cater for the inevitably inconstant quality.

[0008] When the headrest guide-tubes to be made from welded steel tubing, it is also known for the guide-tubes to be fixed into the top-rail of the seat-frame, not by welding, but by lock-beading Lock-beading the guide-tubes to the frame is a considerable improvement over welding the guide-tubes to the frame. In lock-beading, a first bead is produced by pressing the tube axially between dies, whereby a section of the wall of the tube bulges outwards. The tube is placed in a prepared hole in the top-rails of the seat-frame, and then a second bead is pressed into the tube, trapping and locking the rail between the two beads. The use of lock-beading in a headrest guide-tube context is described in patent publication U.S. Pat. No. 6,035,516. The present invention also makes use of the lock-bead technology.

[0009] It is also known, generally, to make a tubular article by bending or rolling a flat piece of sheet metal into a cylinder. The edges of the sheet abut together at a join. In the present case, it may be contemplated to make the headrest guide-tube by folding a piece of flat sheet metal into the basically tubular form of the guide-tube; however, it is recognised that, since the edges of the metal, at the join, are open and unconfined, the necessary rigidity and dimensional accuracy required in the guide-tube cannot be achieved if the guide-tube is made simply by rolling up a flat sheet into a cylinder.

[0010] In it also known, in the automotive field and elsewhere, to provide a series of tabs and sockets along both edges of the flat sheet. When the sheet is rolled up to make a cylindrical tube, the tabs and sockets engage and become interlaced, and become locked together. The interlaced tabs and sockets make the resulting tube considerably more rigid, and dimensionally more stable, than a tube in which the edges of the sheet simply abut together. Tabs and sockets that dovetail together, to interlace two edges together, are shown e.g in U.S. Pat. Nos. 5,239,888 and 1,999,818.

[0011] A tube formed with a well-designed interlaced tabs-and-sockets edge-join can perform, in many respects, and particularly as regards mechanical rigidity and strength, almost as well as a tube made of continuous solid metal. A tube with interlaced tabs and sockets is much more able to accurately retain dimensions, particularly diametral dimensions, than a tube formed by simply bending the metal into a tube, with no interlacing at the abutting edges.

[0012] However, it is recognised that lock-beading and interlaced tabs-and-sockets are difficult features to combine. If it were to be attempted to form rings or beads in a tube made of sheet metal, whose edges were held together with interlaced tabs and sockets, the form of the bead, at the point of interlacing, might be ill-defined. It might happen that there would be no problem, and the metal would distort and fold, at the point of interlacing, in more or less the same way as the metal distorts and folds as the bead is formed over the rest of the circumference of the tube; on the other hand, the chance is too high that the lock-beading operation might form the tabs and sockets into odd promontories or fingers of metal. If such fingers were to become twisted or caught the wrong way in the dies, that might damage the dies; or the tube might pass through the dies, with the finger in a detrimental position, inside the tube, which might make it impossible, later, for the plastic sleeve to be inserted into the guide-tube. Furthermore, it would be difficult to determine, by inspection, whether such an finger had been formed in a detrimental position; possibly, the ring-bead might look good from the outside, where a finger had been left protruding inside.

[0013] It is recognised that, if the guide-tube is to be attached to the seat-frame by lock-beading, the lock-beading should be perfect. The aim should be for the lock-beading to be done so well that a lock-bead that looks acceptable from the outside can be relied upon to be perfect on the inside.

[0014] It is recognised, in the invention, that lock-beading can still be used, as the means by which the guide-tube is secured into the seat-frame, and still the tube can be made from a piece of flat metal, bent into a cylinder, where the edges thereof are interlocked together by interlaced tabs and sockets.

[0015] In the invention, the tabs and sockets appear only in certain portions of the line of the abutting edges of the metal. In the portion of the line of the abutting edges where the beads are to be formed, the edges are plain, i.e do not contain interlaced tabs and sockets.

[0016] Thus, the invention makes use of what may be termed interrupted interlacing. It is recognised, in the invention, that a headrest guide-tube is just the right size and configuration to undergo interrupted interlacing, and still be strong and rigid enough in the areas where strength and rigidity are required. It is recognised that headrest guide-tube is highly suitable for tabs-and-sockets interlacing, as to the diameter and length of the tube, and as to material thickness, (assuming the tube is made of steel). It is recognised that the forces and stresses that an automobile headrest guide-tube is subjected to, in operation, can be accommodated by an only-partially-interlaced tube. It is recognised that the configuration of a headrest guide-tube is such that the disposition and number of tabs and sockets needed for partial or interrupted can easily and conveniently be provided.

[0017] In the invention, a headrest guide-tube is made from a piece of flat sheet metal, bent over into the form of a tube, in which the upper, and preferably also the lower, portions of the side-edges of the sheet, upon being pressed together, become interlaced; but the ring-beads, by which the guide-tube is attached into the top-rail of the seat-frame, are formed in the middle-portion of the tube, where the side-edges are plain, and simply abut together without interlacing.

[0018] The designer should pay attention to the shapes of the tabs and sockets, to ensure that, as the folded-over edges of the sheet start to interlace, the tabs and sockets can engage together without interfering. Some practical standards have been worked out, in the metal-forming industry, as to the most advantageous shapes and configurations for the interlocking tabs and sockets, and the designer should follow those standards in the present case. That is to say, in the invention, the portions of the tube that have interlaced edges should have those edges interlaced as well as possible; where the ring-beads are located, i.e in the middle-portion where the edges are not interlaced, the edges may simply abut, or may even be spaced slightly apart.

[0019] It is recognised that a headrest guide-tube loses very little, from the standpoints of mechanical strength, dimensional integrity, and the ability generally to perform as a headrest guide-tube, by having no interlacing over the middle-portion of the tube.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] By way of further explanation of the invention, exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0021]FIG. 1 is a pictorial view of a headrest guide-tube for an automobile seat, which embodies the invention.

[0022]FIG. 2 is a diagram showing the piece of sheet metal from which the guide-tube of FIG. 1 was made.

[0023]FIG. 3 is a side-view of the guide-tube of FIG. 1 at an intermediate stage of manufacture.

[0024]FIG. 4 is a side-view of the guide-tube of FIG. 1, in dies, at a later stage of manufacture.

[0025]FIG. 5 is a cross-section of the guide-tube of FIG. 1, installed into the frame of an automobile seat.

[0026]FIG. 6 is a side-view of the guide-tube of FIG. 1, in dies, at a still later stage of manufacture.

[0027]FIG. 7 is a plan view of the guide-tube of FIG. 1.

[0028] The apparatuses shown in the accompanying drawings and described below are examples which embody the invention. It should be noted that the scope of the invention is defined by the accompanying claims, and not necessarily by specific features of exemplary embodiments.

[0029] The tube 20 shown in FIG. 1 comprises a piece of sheet steel, which has been bent around, from a flat sheet, into the cylindrical tubular form as shown. The tube 20 is securely locked into the tubular cylindrical configuration by means of interlaced tabs and sockets, by means of which the bent-over side-edges of the sheet are stitched together. Headrest guide-tubes typically have a wall-thickness of about 1½ mm, and an internal diameter of about 16 mm. Typically, the guide-tubes have an axial length of 7 or 8 cm.

[0030] As shown in FIG. 2, tabs 23 and sockets 24 are formed in the left and right edges 25L,25R of the flat piece 26 of sheet metal from which the tube 20 is to be formed. The designer ensures that the tabs 23 in the right edge 25R correspond to the sockets 24 in the left edge 25L, and vice versa. As shown, all the tabs are the same and all the sockets are the same, but this is not essential. The tabs as shown are roughly 4 mm square. The upper and lower edges of the tabs are not straight, but are shaped for easy dovetailing of the tabs into the corresponding sockets, and for secure locking once interlacing is complete.

[0031] The tabs 23 and sockets 24 appear in the upper-portion 27L and in the lower-portion 28L of the left edge 25L of the sheet 26, and similarly in the upper-portion 27R and in the lower-portion 28R of the right edge 25R. But the central portions 29L,29R of the left and right edges 25L,25R are straight; that is to say, there are no tabs and sockets over the central portions of the edges.

[0032]FIG. 3 shows the tubular shape to which the piece 26 of sheet metal is first formed. The flat sheet metal 26 of FIG. 2 is bent to a U-shape, between a cylindrical tool and a die (not shown) of half-round concave shape. Then, a complementary half-round die (not shown) presses over the arms of the U, and drives the tips of the arms together, until the sheet of metal encircles the cylindrical tool. As bending progresses, the tabs 23 start to engage into the sockets 24, and the left and right edges 25L,25R of the sheet metal become progressively interlaced.

[0033] As the two half-round dies are brought together, the thickness of the sheet metal is trapped between the half-round dies and the cylindrical tool. A final heavy squeeze-pressure on the dies coins the metal into the tubular form illustrated in FIG. 3.

[0034] Next, a ring-bead is formed on the tube. This is done by pressing the tube in an axial direction between shaped dies 30,32, as shown in FIGS. 4 and 4a. A recess 34 in one of the dies allows metal to expand radially outwards as the tube is pressed axially, and the walls buckle, thereby forming the ring-bead 35.

[0035] The formed ring-bead 35 comprises an upper section 36 of the wall of the guide-tube, and a lower section 37.

[0036] The designer may prefer to have the dies 30,32 bottom together, or may prefer to leave a gap 38, which ensures that, as the press-stroke is completed, the full pressure of the press acts to coin together the upper and lower sections 36,37 of the tube wall that form the ring-bead.

[0037] The tube 20, with the single bead 35 as formed thereon, is as shown in FIG. 1, and is now ready to be inserted into the seat-frame. This operation might be done with the FIG. 1 tube still in the same automatic handling facilities with which the tube was taken through the press as in FIG. 4; or, the FIG. 1 tubes might be packed in boxes and transported to a separate factory for assembly into the seat-frames.

[0038]FIG. 5 shows a guide-tube assembled to a section of a seat-frame 39. In this case, the seat-frame comprises a length of extruded I-section aluminum, which is bent and shaped to define the sides and top-rail of the seat. The seat-frame may alternatively comprise a length of round tubing, which is squashed flat and through-pierced at the point where the headrest guide-tubes are to be inserted. The invention is not limited to the particular manner of forming the seat-frame; provided the guide-tubes can be lock-beaded into the seat-frame, the invention may be applied.

[0039] Two of the FIG. 1 tubes are inserted into respective holes formed in the top-rail 40 of the seat-frame, the holes being punched through the web of the I-section. Then, the tubes are lock-beaded into the seat-frame.

[0040] Lock-beading is done by inserting the FIG. 1 tube 20 into the pierced hole in the seat-frame, and then pressing the tube axially. The tube is confined by dies 42,43 as shown in FIG. 6, in which the die 43 is shaped, as at 45, to permit the wall of the tube again to deflect radially outwards, and thus to form the second ring-bead or lock-bead 46. The designer preferably should ensure that the dies 42,43 do not bottom against each other, but rather that the four wall thicknesses that form the two ring-beads are all squeeze-pressed together over the web of the seat-frame. The headrest guide-tube 49 is now fully formed, and fully secured into the seat-frame. No further processing is necessary.

[0041]FIG. 7 is a plan view of the headrest guide-tube 49, showing the first ring-bead 35. (It will be understood that the lock-bead 46, formed on the other side of the web of the seat-frame, is of the same configuration.) In the sector adjacent to the vertical join 47, the bead 35 is only imperfectly formed. That is to say, the ring-bead does not extend outwards quite to the same radius as the radius to which the ring-bead expands over the remainder of the circumference of the guide-tube.

[0042] The fact that the ring-bead, in this adjacent-to-the-join sector, is only-partially formed has substantially no effect on the security with which the guide-tube is retained in the seat-frame. Indeed, the fact that the bead differs, as to its form, over different sectors of its circumference, means that the bead is non-round; this non-roundness is advantageous because, when the tube is pressed into the seat-frame, the non-roundness is enough to lock the guide-tube against rotating in its hole in the seat-frame. If the tube and the ring-beads were uniform at all points around the circumference, the only factor preventing the headrest guide-tube from rotating in the hole in the frame would just be the tightness by which the beads locked onto the web of the frame. Theoretically, that is enough; but, at the extremes of tolerance, such mere tightness might not quite be enough. The problem is that, once the headrest guide-tube can rotate, even slightly, in the seat-frame, a very great tendency then arises for the guide-tube to start to work loose. The out-of-roundness as shown in FIG. 7 alleviates this problem.

[0043] The shape of the ring-bead 35 adjacent to the join 47, as shown in the FIG. 7 plan view, may be understood as providing a smooth transition 48 from virtually no bead, at the edges of the join, to the full bead shape as shown in the (FIG. 4) sectional view. Not only is it advantageous that this transition or lead-in to the full bead should be gradual, and smooth, but also the form of the transition ensures there are no protruding sharp edges at the transition, which might injure the persons working on the seats, or might damage the seat material.

[0044] Given that the bead 35 is imperfectly formed at the edges of the join 47, it is noted that the imperfection does not give rise to any tendency for metal to appear in places where it might cause a problem. For example, the imperfection does not appear as an unwanted tag or finger or other protrusion of metal that might inhibit the dies from closing properly, or might cause damage or injury. Rather, the imperfection is manifested as a smooth transition 48.

[0045] As mentioned, the imperfection of the shape of the ring-bead adjacent to the join does not give rise to sharp edges, and does not affect the security of the connection between the guide-tube and the seat-frame. Indeed, the imperfection gives rise to a non-roundness that enhances that security.

[0046] The advantageous lead-in or transition 48 to the full form of the ring-bead 35, as shown in the plan view of FIG. 7, arises because the central portions 29L,29R of the edges 25L,25R of the metal, at the join 47, are straight. This may be contrasted with what the plan view of FIG. 7 would look like if the tabs 23 and sockets 24 were to extend over the full length of the edges 25L,25R of the piece of sheet metal. If the axially-pressed circumferentially-buckled ring-bead were to be formed in an area where there are tabs and sockets, the designer could not be sure whether, in the plan view of FIG. 7, there might not be ragged or protruding fingers of metal. Depending on the position of the bead relative to the tabs and sockets, such fingers might protrude inwards or outwards.

[0047] Such protruding fingers might, as mentioned, become trapped in the dies, or might cause injury or damage. The consequence of trapping an extra thickness of metal in the die is not only that it might damage the die, which is bad enough, but that the tube with the trapped finger might pass through the die, and be assembled into the seat-frame. It can be difficult to detect that a ring-bead is improperly-formed on the inside.

[0048] It is recognised that the designer should not contemplate pressing a ring-bead into the tube, if the edges 25L,25R of the metal at the join 47 were to be anything other than plain. Preferably, the edges should also be straight, or nearly straight. Forming a ring-bead should not be contemplated if the edges have tabs and sockets right where the ring-bead (i.e either of the two ring-beads) is to be formed. It is not essential that the line of the join be vertical (the line of the join may have a (slight) helical component, for example), but the line of the join, in the area at which the beads are to be formed, should not have any abrupt changes of direction, and should not define any shapes that might turn into protruding fingers when the beads are formed.

[0049] The headrest guide-tube 20 is provided with slots or a slot 50 in its upper end. Some aspects of these slots will now be described. In a headrest guide-tube, the component that fits immediately inside the guide-tube is a plastic sleeve. (The metal post of the headrest itself fits inside that plastic sleeve.) The slots in the guide-tube are provided for the purpose of aligning the plastic sleeve into its correct orientation relative to the guide-tube. The plastic sleeve has moulded pegs, which engage in the slots.

[0050] When the headrest guide-tubes are formed from lengths of pre-welded tubing, cutting the slots into the tubing can be a major problem. Forming the slots 50 is much simpler when the headrest guide-tubes are formed by rolling the tubes from flat sheets, because then it is simple enough to stamp out the form of the slots while the guide-tube is still in the form of a flat piece of sheet metal 26. When the slots were cut in pre-welded tube, by contrast, the slot-cutting tool left burrs and sharp edges, the removal of which can be a difficult production operation; if the edges of the slots were left very sharp, the moulded pegs on the plastic sleeves might be cut or damaged as the production personnel manoeuvre the sleeves into the guide-tubes. But now, when the slots 50 are stamped out of the flat sheet metal, it is easy to give the corners of the slots a slight chamfer, as at 52.

[0051] Also, when the slot was cut in pre-welded tube stock, the slot might happen to coincide with the line of the weld, and in that case the slot might be improperly formed.

[0052] Also, if ring-beads were formed in pre-welded tube stock, the buckling that occurs at the ring-bead might cause the weld to burst. This might lead, if not to slivers of metal being trapped inside the ring bead, at least to jagged edges. Ring-beads formed in a tube in which the edges simply abut together, on the other hand, have the advantageous form as shown in FIG. 7.

[0053] As described, a tube that has interlaced edges can be indistinguishable, as regards mechanical strength and dimensional accuracy, from a continuous-metal tube. Furthermore, as shown in FIG. 4, the hollow interiors 53,54 of the dies 30,32, which receive the axial ends of the guide-tube, may be tapered slightly, to swage the upper and lower end-portions of the tube inwards. Such a measure gives better control over the dimensional accuracy of the diameters of the tube adjacent to the ends of the tube. The middle-portion 29 of the tube, where the ring-beads are formed, can be left without being swaged, since diametral accuracy over the middle-portion is not important; the ring-bead formations are, in any event, driven into tight intimate contact with the metal of the top-rail of the seat-frame, even if the hole in the top-rail is inaccurately formed.

[0054] That is to say, the goodness of the fit of the plastic sleeve into the guide-tube is determined by the accuracy of the diameter in the upper and lower (interlaced) portions of the guide-tube, and the swaging enables that fit to be accurately controlled. (It may be noted that, with pre-welded tube, the weld sometimes left a slight ridge inside the tube, which could interfere with the fit of the plastic sleeve in the guide-tube.) In the plain, non-interlaced, middle-portion of the guide-tube, leaving the guide-tube with an accurate internal diameter is not so important; in the middle-portion, the operation of lock-beading the guide-tube into the top-rail produces a high-integrity joint, irrespective of whether the diameter of the through-hole in the top-rail was quite accurate. In fact, punching through-holes in the rails of seat-frames should be regarded as a rather low-accuracy operation; one reason lock-beading is so well regarded for headrest guide-tubes is that lock-beading does not need the through-hole in the top-rail to be particularly accurate. 

1. Headrest guide-tube for an automobile seat, wherein: the tube has walls, which comprise a single sheet of metal, having left and right side-edges; the walls comprise the sheet metal in a bent-over condition, in which the sheet has the configuration of a cylindrical tube; the left and right side-edges of the sheet include respective upper-, middle-, and lower-, portions; the upper-portion of the left side-edge carries a series of tabs and sockets, and the upper-portion of the right side-edge carries a corresponding series of sockets and tabs; the bent-over tube is of a configuration in which the series of tabs and sockets carried on the upper-portion of the left side-edge lie interlaced with the series of sockets and tabs carried on the upper-portion of the right side-edge, thereby defining an interlaced upper-portion of the tube; the middle-portions of the left and right side-edges of the sheet are plain and substantially free of tabs and sockets, whereby, in the bent-over configuration of the tube, the middle-portions of the side-edges are not interlaced, thereby defining a non-interlaced middle-portion of the tube; the tube includes a first ring-bead, in which the sheet metal wall of the tube lies buckled radially outwards; and the first ring-bead lies in the non-interlaced middle-portion of the tube.
 2. Guide-tube of claim 1, wherein: the lower-portion of the left side-edge carries a series of tabs and sockets, and the lower-portion of the right side-edge carries a corresponding series of sockets and tabs; the bent-over tube is of a configuration in which the series of tabs and sockets carried on the lower-portion of the left side-edge lie interlaced with the series of sockets and tabs carried on the lower-portion of the right side-edge, thereby defining an interlaced lower-portion of the tube.
 3. Guide-tube of claim 1, in combination with a seat-frame having a top-rail, wherein: the tube passes through a through-hole in the top-rail, and the first ring-bead lies in contact with one side of the top-rail; the tube includes a second ring-bead, in which the sheet metal wall of the tube lies buckled radially outwards; the second ring-bead also lies in the non-interlaced middle-portion of the tube; the second ring-bead lies on the opposite side of the top-rail from the first ring-bead, and the top-rail lies gripped between the first and second ring-beads.
 4. Procedure for manufacturing a headrest guide-tube for an automobile seat, including: providing a piece of sheet metal, having left and right side-edges; providing, along an upper-portion of the left side-edge of the piece of sheet metal, a series of tabs and sockets; providing, along an upper-portion of the right side-edge, a corresponding series of sockets and tabs; the series of tabs and sockets on the left side-edge corresponds to the series of sockets and tabs on the right side-edge in that, when the side-edges are brought together, the upper-portions of the left and right side-edges become interlaced together; providing, along a middle-portion of the left side-edge, a left plain-edge, free of tabs and sockets; providing, along a middle-portion of the right side-edge, a corresponding right plain-edge, free of tabs and sockets; bending the piece of sheet metal into the form of a cylindrical tube; the tube, thus formed, includes an upper-portion of the tube in which the upper-portions of the side-edges are interlaced together, and includes a middle-portion of the tube in which the middle-portions of the side-edges are not interlaced together; placing the tube between dies; the dies include a middle-portion of the dies, and the middle-portion of the dies is formed with a recess, into which the metal of the tube can expand radially outwards; the recess is so positioned that only metal from the middle-portion of the tube, in which the edges are not interlaced, can enter the recess; compressing the cylindrical tube axially with enough force to cause the metal of the tube to buckle axially, and expand outwards radially, into the recess, thereby forming a first ring-bead in the middle-portion of the tube.
 5. Procedure of claim 4, wherein the dies include an upper-portion of the dies, which is so arranged as to confine the said interlaced upper-portion of the tube against radially-outward expansion during axial compression of the tube.
 6. Procedure of claim 5, wherein the upper-portion of the dies is smaller, diametrally, than the interlaced upper-portion of the tube, whereby compressing the tube axially between the dies is effective to swage the upper-portion of the tube.
 7. Procedure of claim 4, including: providing, along a lower-portion of the left side-edge of the piece of sheet metal, a lower series of tabs and sockets; providing, along a lower-portion of the right side-edge, a corresponding lower series of sockets and tabs; the lower series of tabs and sockets on the left side-edge corresponds to the lower series of sockets and tabs on the right side-edge in that, when the side-edges are brought together, the lower-portions of the left and right side-edges become interlaced together; the tube, thus formed, includes a lower-portion of the tube in which the lower-portions of the side-edges are interlaced together.
 8. Procedure of claim 7, wherein the dies include a lower-portion of the dies, which is so arranged as to confine the said interlaced lower-portion of the tube against radially-outward expansion during axial compression of the tube.
 9. Procedure of claim 7, wherein the lower-portion of the dies is smaller, diametrally, than the interlaced lower-portion of the tube, whereby compressing the tube axially between the dies is effective to swage the lower-portion of the tube.
 10. Procedure of claim 4, including: placing the tube in a through-hole in a top-rail of a seat-frame, with the first ring-bead abutting against the top-rail, and against the edges of the hole therein; placing the top-rail and the tube, together, between dies, where the dies include a second recess into which the wall of the tube can expand outwards upon being compressed axially; the second recess in the dies is so positioned that only metal from the middle-portion of the tube, in which the edges are not interlaced, can enter the second recess; the second recess is positioned on the opposite side of the top-rail from the first ring-bead; compressing the tube axially with enough force to cause the metal of the tube to buckle axially, and expand outwards radially, into the second recess, thereby forming a second ring-bead in the middle-portion of the tube; whereby the top-rail is left gripped between the first and second ring-beads.
 11. Procedure of claim 4, wherein the sheet has a top edge, and the procedure includes forming a slot in the top edge prior to bending the piece of sheet metal into the form of the cylindrical tube.
 12. Procedure of claim 11, including forming chamfers at the entrance, in the top edge, into the slot. 